1. Field of the Invention
The present invention relates to a method of loading fibers in a fiber suspension for use in a paper-making machine with a chemical compound, and, more particularly, to a method for loading fibers in a fiber suspension with calcium carbonate.
2. Description of the Related Art
A paper-making machine receives a fiber suspension including a plurality of fibers, such as wood fibers, which are suspended within an aqueous solution. The water is drained from the fiber suspension and dried in the paper-making machine to increase the fiber content and thereby produce a fiber web as an end product.
The fiber web produced by the paper-making machine typically includes organic wood fibers and inorganic fillers. A known inorganic filler is calcium carbonate, which may be added directly to the fiber suspension (direct loaded calcium carbonate). It is also known to chemically load the fibers within a fiber suspension with calcium carbonate in the lumen and walls of the individual fibers (fiber loaded calcium carbonate). The fiber loaded calcium carbonate increases the strength of the paper compared with a direct loaded calcium carbonate (adding calcium carbonate directly to the fiber suspension) at the same loading (filler) level. This yields an economic advantage in that the filler level of the paper is increased by replacing the more expensive fiber source (wood fibers) with calcium carbonate. The finished paper web has higher strength properties due to the increased filler levels of the calcium carbonate. In contrast, the strength properties of a finished web using direct loaded calcium carbonate is less.
For example, U.S. Pat. No. 5,223,090 (Klungness, et al.) discloses a method for chemically loading a fiber suspension with calcium carbonate. In one described method, calcium oxide or calcium hydroxide is placed within a refiner unit and carbon dioxide is injected into the refiner unit at a specified pressure. The fiber suspension is maintained within the refiner for a predetermined period of time to ensure that a proper chemical reaction and thus proper chemical loading of the fiber suspension occurs. In another described method, a fiber suspension with calcium oxide or calcium hydroxide is introduced into a 20 quart food mixer and carbon dioxide gas is injected into the mixer at a specified pressure. Using either the refiner or the food mixer, both methods utilize a batch processing method for processing only a small amount of the fiber suspension at a time. Because of the large amount of fiber suspension which is required at the wet end of a paper-making machine, a batch process requires that the chemically loaded fiber suspension be transferred to another holding tank for ultimate use in a paper-making machine.
What is needed in the art is a method for chemically loading calcium carbonate in and on fibers in a fiber suspension for use in a paper-making machine, which allows commercialization of such a chemical loading process, and which allows the physical properties of the fiber web to be altered.
The present invention provides a fiber loading apparatus which effectively loads fibers within a fiber suspension, and which is compactly constructed and arranged to occupy less physical space.
The invention comprises, in one form thereof, a method of continuously loading fibers in a fiber suspension with calcium carbonate. The fibers include a fiber wall surrounding a lumen. A reactant solid in the form of calcium oxide and/or calcium hydroxide is mixed into the fiber suspension with a resultant initial process pH of between 11 and 12. The fiber suspension is transported at a consistency of between approximately 15 and 30% into an inner chamber of a closed reactor. A reactant gas is injected into the reactor, whereby the reactor is pressurized to a pressure between 5 and 150 psi. A temperature of the fiber suspension within the reactor is controlled at a range between xe2x88x9210xc2x0 C. and 80xc2x0 C. The fibers within the fiber suspension are loaded with calcium carbonate as a result of a chemical reaction between the reactant solid and the reactant gas in the reactor over a predetermined reaction time. A specific type of calcium carbonate crystals are grown on the fiber walls of the fibers, depending upon the initial process pH, temperature, pressure and reaction time.
An advantage of the present invention is that the fiber loading of the fiber in the fiber suspension takes place as a continuous process, thereby providing output quantities of loaded fiber suspension sufficient for use in a paper-making machine.
Another advantage is that variables such as flow rate, temperature and pressure which affect the fiber loading process can be accommodated and varied.
Yet another advantage is that specific types of calcium carbonate crystals are grown on the fiber walls of the individual fibers, thereby providing different physical properties to the fiber web produced as an end product.